The present invention relates to a position measuring apparatus which measures position information of a mark formed on an object such as a wafer or glass plate, or a mask or reticle, in a manufacturing process of semiconductor devices, liquid crystal display devices or the like, and an exposure apparatus which carries out alignment of an object, using the position information of the mark obtained by the position measuring apparatus, to expose a pattern formed on the mask or the reticle onto the wafer or the glass plate.
In manufacturing of devices such as semiconductor devices or liquid crystal display devices, an exposure apparatus is used to repetitively perform projection and exposure of a fine pattern image formed on a photo-mask or a reticle (hereinafter generally referred to as a xe2x80x9creticlexe2x80x9d) onto a substrate such as a semiconductor wafer or a glass plate, to which a photosensitizer such as a photo resist has been applied. When projection exposure is to be performed, it is necessary to precisely align the position of the substrate and the position of a pattern image formed on the reticle. The exposure apparatus has an alignment device for performing this alignment. The alignment device comprises an alignment sensor for detecting the position of an alignment mark formed on the substrate, and a control system for performing alignment of the substrate based on the position of the alignment mark detected by the alignment sensor.
Since the surface condition (roughness level) of the substrate, being an object to be measured, changes in the manufacturing process of semiconductor devices or liquid crystal display devices, it is difficult to accurately detect the position of the substrate by one alignment sensor. Therefore, a different sensor is generally used according to the surface condition of the substrate. Main alignment sensors include an LSA (Laser Step Alignment) type, an FIA (Field Image Alignment) type, and an LIA (Laser Interferometric Alignment) type. The outline of these alignment sensors will be described below.
The LSA type alignment sensor is an alignment sensor which irradiates a laser beam onto an alignment mark formed on the substrate, and measures the position of the alignment mark, using the diffracted and scattered light, and has been heretofore widely used for semiconductor wafers in various manufacturing steps. The FIA type alignment sensor is one which performs position measurement by illuminating an alignment mark, using a light source having a wide wavelength bandwidth such as a halogen lamp, and performs image processing of the image of the alignment mark obtained by the illumination result, and is effective for measurement of asymmetric marks formed on an aluminum layer or on the surface of a substrate. The LIA type alignment sensor is one which irradiates laser beams having a slightly different wavelength from two directions, makes the two diffracted light generated as a result thereof interfere with each other, and detects the position information of the alignment mark from the phase of the interfered light. This LIA type alignment sensor is effective, when used for alignment marks having a low difference in level, or a substrate having a large surface roughness.
The position information detector includes a TTL (Through The Lens) type which detects the position information of a mark on a substrate via a projection optical system, an off-axis type which directly detects the position information of a mark on a substrate, without using the projection optical system, and a TTR (Through The Reticle) type which observes a substrate and a reticle at the same time via the projection optical system, and detects the relative position thereof. When alignment of the reticle and the substrate is performed by using these position information detectors, a baseline quantity, which is a spacing between the measurement center of the position information detector and the center (exposure center) of the projected image of a pattern on the reticle, is determined in advance. Then the amount of misalignment of the mark from the measurement center is detected by the position information detector, and the substrate is shifted by a distance obtained by correcting this amount of misalignment by the baseline quantity, to thereby accurately align the center of a section area (shot area) set on the substrate with the exposure center. The shot area is then exposed by the exposure light. The baseline quantity may change gradually in the process of holding and using the exposure apparatus. If a so-called baseline change, being a change in the baseline quantity, occurs, the alignment accuracy (superposition accuracy) decreases. Therefore, it is necessary to regularly carry out a baseline check for accurately measuring the spacing between the measurement center of the position information detector and the exposure center.
One example of the overall operation of the exposure apparatus will be outlined below.
Before a substrate is carried to the exposure apparatus, the position information of a mark formed on a reticle is detected by a reticle position information detector, and position adjustment of the reticle is performed based on the position information. The substrate is then carried to the exposure apparatus, and the position information of a mark formed on the substrate is detected by a substrate position information detector. The substrate is then shifted by a distance obtained by correcting the amount of misalignment indicated by the position information of the substrate by the baseline quantity, within a plane perpendicular to the optical axis of the exposure light, based on the position information of the mark formed on the substrate, to thereby align the relative positions of the reticle and the shot area formed on the substrate. After this, and the exposure light is irradiated onto the reticle, to expose on the substrate the image of a pattern formed on the reticle.
Incidentally, the mark formed on the substrate is, for example, as shown in FIG. 12. FIG. 12 is a diagram showing one example of a mark formed on the substrate for position measurement. In FIG. 12, a mark 100 is one where rectangular mark elements 101 having a longitudinal direction are arranged substantially parallel in the longitudinal direction of each mark element 101, with a predetermined interval, for example, several xcexcms, in a direction orthogonal to the longitudinal direction. Therefore, the mark 100 shown in FIG. 12 has a construction such that the surface position changes periodically with respect to the direction orthogonal to the longitudinal direction of the mark elements 101, that is, in the direction indicated by reference symbol 102 in the figure.
The alignment sensor detects periodical changes of the surface position to measure the position information of the mark 100. For example, the FIA type alignment sensor detects an edge position of the mark elements 101, by performing image processing with respect to image information in which a signal strength (brightness of the image) changes according to the periodic change of the surface position, and measures the position information of the mark 100 (for example, position information indicating the central position of the mark 100), based on the detected edge position. When the position information of the mark 100 is to be measured by performing image processing, if image information having a sufficient strength cannot be obtained, the position information cannot be measured with high accuracy. Therefore, amplification is carried out by an AGC (Automatic Gain Control) circuit or the like, and it is set such that the strength of the image information so as to become a strength within a certain range.
Measurement processing of the position information by the FIA type alignment sensor will be described in detail below. FIG. 13 is a diagram for explaining the position information measurement processing by the FIA type alignment sensor. The FIA type alignment sensor includes an image pickup device constituted by arranging a plurality of pixels 103 on an image plane, that is, on an image pickup plane 104 of a mark image. As the image pickup device, for example, a CCD (Charge Coupled Device) is used. In FIG. 13, for easy understanding, there is shown the situation where images 110, 111 and 112 of three mark elements, of the mark elements 103 shown in FIG. 12, are formed on the image pickup plane 104. The pixels arranged on the image pickup plane 104 are for receiving the incident light and converting it into an electric signal. The image pickup device converts an image entering into the image pickup plane into image information, by sequentially scanning the arranged pixels.
That is to say, as shown in FIG. 13, pixels arranged in row r1 are sequentially scanned in the scanning direction indicated by reference symbol 105 in the figure. When scanning is finished for all elements arranged in row r1, pixels in the direction orthogonal to the scanning direction 105 indicated by reference symbol 106 in the figure and arranged in row r2, are sequentially scanned in the scanning direction 105, and in this manner, pixels arranged in row r3, r4, . . . are sequentially scanned. Of the image information obtained by scanning in this manner, the image information obtained by scanning row r1 is output as image information C1, and the image information obtained by scanning row r2 is output as image information C2. Image information is similarly output for the other rows. In FIG. 13, for easy understanding, the number of pixels arranged on the image pickup plane is shown decreased.
Generally, when the position information of the mark 100 is to be measured, the longitudinal direction of the mark elements 101 forming the mark 100 is set so as to be orthogonal to the scanning direction of the image pickup device. Incidentally, even if the strength of the image information detected by the image pickup device changes for each mark, the strength of the image information is stabilized and set to a value within a certain range, by providing the above-described AGC circuit. However, the image information of the mark is amplified at an amplification factor which is different for each mark, and as a result, misalignment error occurs in the measured position information. Next an explanation is given for the reason why misalignment errors occur due to the difference of the amplification factor for the AGC circuit.
FIG. 14 is a diagram showing an example of results of amplifying the image information, obtained by converting the images 110, 111 and 112 of the mark elements shown in FIG. 13 by the image pickup device, at a differ amplification factor. In FIG. 14, curves denoted by reference symbols d1, d2 and d3 show a part of the image information obtained by amplifying the image information at different amplification factors, respectively. The set amplification factors increase in order of the curves d1, d2 and d3. The image information shown in FIG. 14 indicates the image information obtained by using the above described image pickup device, which performs scanning processing timewise, wherein time is plotted on the X axis and the signal strength is plotted on the Y axis. In FIG. 14, three X axes are shown, and the points in time when the lines set parallel with the Y axis intersect each X axis are the same points in time.
As is seen from FIG. 14, the detection signal obtained by amplifying the signal at a different amplification factor is delayed timewise. This is because the frequency characteristic of the amplifying circuit including the AGC circuit deteriorates with respect to high frequency components. That is to say, when Fourier transformation is applied to the detection signal shown in FIG. 14, this can be separated into each frequency component. Since the amplifying circuit has generally a small gain with respect to high frequency components, low frequency components can be amplified by a set amplification factor, but high frequency components are not amplified at the amplification factor set with respect to the low frequency components.
Thus, when frequency components amplified at a different amplification factor are combined, the signal waveform becomes dull, as shown by curves d2 and d3 in FIG. 14. This tendency generally becomes conspicuous, as the amplification factor set with respect to the detection signal becomes high. As shown in FIG. 14, when the detection signal is set on the time base, the position information of the mark is measured, based on the position of the detection signal on the time base, for example, based on the position on the time base of a maximal part and a minimal part. Therefore, as shown in FIG. 14, en the amplification factor is different, the detection signal becomes dull, and as a result, the position of the detection signal on the time base changes. The change quantity of the position of the detection signal on the time base also changes, when the amplification factor is different.
As shown in FIG. 13, the image pickup device performs scanning of the arranged elements to obtain image information. However, since scanning is a processing for obtaining image information of pixels arranged at positions different in a time series, the image information obtained by scanning is a signal that changes with time. Moreover, the mark 100 shown in FIG. 12 is for measuring position information indicated by reference symbol 102 in FIG. 12. As described above, when the position information of the mark 100 is to be measured, the longitudinal direction of the mark elements 101 forming the mark 100 is set so as to be orthogonal to the scanning direction of the image pickup device. That is to say, when the amplification factor is changed and the signal waveform of the image information becomes dull, the position of the mark 100 on the time base becomes a position delayed timewise, and as a result, the measured position information is deviated from the original position of the mark 100. Further, with a change of the amplification factor, the amount of misalignment also changes.
It is considered that there is no problem if the amount of misalignment of the mark is not larger than the resolving power required at the time of alignment. Therefore, it has also been considered to improve the AGC circuit and to design the AGC circuit so that the misalignment hardly occurs, or the amount of misalignment is not larger than the resolving power required at the time of alignment, However, when the size of the visual field of the position measuring apparatus is about 200 xcexcmxc3x97160 xcexcm, and the number of pixels of the image pickup device is 640xc3x97480 pixels, one pixel corresponds to a distance of about 0.3 xcexcm on the substrate. The resolving power generally required at the time of alignment is about 10 nm, and hence this distance is about {fraction (1/30)} of the above one pixel. If the distance of 10 nm is converted into the time interval in the image information, it corresponds to about 3 ns, which is a very short time as a delay for the electric signal. Accordingly, it is very difficult to design an AGC circuit in which misalignment does not occur, by improving the frequency characteristic.
Recently, higher densities are required in integrated circuits. For example, in a CPU (central control unit) used in personal computers, line widths of 0.18 xcexcm are being put to practical use, but in the near future, there is proposed a plan to make the line width 0.1 xcexcm. Therefore, it is considered that fine processing techniques with higher densities will be required in the future. In order to respond to such high density requirement, it becomes very important to reduce the measurement error in the position information to as small as possible, and to improve alignment accuracy by performing alignment based on position information measured with high accuracy.
In view of the above situation, it is an object of the present invention to provide a position measuring apparatus which can measure the position information of a mark formed on an object with high accuracy, and an exposure apparatus which can perform exposure by performing alignment highly accurately, based on the highly accurate position information measured by the position measuring apparatus, and as a result, can realize fine processing.
In order to solve the above problems, the position measuring apparatus of the present invention is a position measuring apparatus (14, 18) which measures position information in a predetermined direction (D1, D2, D3) of a mark (AM, AM1) formed on an object (W), which comprises: an irradiation device (15, 16, 20, 21, 24, 25, 26, 27, 28) which irradiates a detection beam (IL2) onto the mark (AM, AM1); an imaging optical system (28, 27, 26, 25, 29, 30, 31, 50) which forms an image (Im1, Im2) of the mark generated from the mark (AM, AM1) due to the irradiation of the detection beam (IL2), on an image plane (F1, F2, F3); an image pickup device (32, 51) which picks up the image (Im1, Im2) of the mark on the image plane (F1, F2, F3), while scanning scanning lines, to generate image information corresponding to the image (Im1, Im2) of the mark; and a calculation device (41) which obtains position information in the predetermined direction (D1, D2, D3) of the mark (AM, AM1), based on the image information, wherein the scanning direction (SC1, SC2, SC3) of the scanning lines is orthogonal to the predetermined direction (D1, D2, D3).
According to this invention, since the predetermined direction of the mark is set so as to be orthogonal to the scanning direction of the scanning lines, even if the image information is amplified, a time lag in the measuring direction does not occur as in the conventional technique, and hence misalignment attributable to the time lag does not occur. As a result, position information of the mark can be measured with high accuracy.
Preferably the position measuring apparatus of the present invention further comprises a memory device (40, 60) which stores the image information, and the calculation device (41) calculates image information of the mark (AM, AM1), based on the image information stored in the memory device (40, 60).
In the position measuring apparatus of the present invention, the image pickup device (32, 51) may be rotatable with respect to the image (Im1) of the mark, so that the direction (SC1) of the scanning lines is orthogonal to the predetermined direction (D1).
In the position measuring apparatus of the present invention, the mark (AM1) may include a first mark (AMY) having periodicity in a first direction (D3), and a second mark (AMX) having periodicity in a second direction (D2) orthogonal to the first direction (D3), and the image pickup device (32, 51) may include a first image pickup device (32) comprising scanning lines extending in a direction orthogonal to the first direction (D3), and a second image pickup device (51) comprising scanning lines extending in a direction orthogonal to the second direction (D2).
In the position measuring apparatus of the present invention, the calculation device (41) may obtain position information of the first mark (AMY) in the first direction (D3), based on the image information obtained by the first image pickup device (32), and may obtain position information of the second mark (AMX) in the second direction (D2), based on the image information obtained by the second image pickup device (51).
The position measuring apparatus of the present invention may have; a branching device (50) which branches the image (Im2) of the mark to an image (ImY) of the first mark and an image (ImX) of the second mark, and guides the image (ImY) of the first mark to the first image pickup device (32), and guides the image (ImX) of the second mark to the second image pickup device (51), a first memory device (40) which stores the image information obtained from the first image pickup device (32), and a second memory device (60) which stores the image information obtained from the second image pickup device (51).
The position measuring apparatus of the present invention is a position measuring apparatus which measures position information in a predetermined direction (the direction of the X axis) of a mark (AM, AM1) formed on an object (W), comprising: an irradiation device (15, 16, 20, 21, 24, 25, 26, 27, 28) which irradiates a detection beam (IL2) onto the mark (AM, AM1); an imaging optical system (28, 27, 26, 25, 29, 30, 31, 50) which forms an image of the mark generated from the mark (AM, AM1) due to the irradiation of the detection beam (IL2), on an image plane (104); an image pickup device (32, 51) which picks up the image of the mark on the image plane (104), while scanning scanning lines, to generate image information corresponding to the image of the mark; and a calculation device (41) which obtains position information in the predetermined direction (the direction of X axis) of the mark (AM, AM1), based on the image information, wherein the image pickup device (32, 51) picks up the image of the mark, while scanning the scanning lines in the predetermined direction (the direction of the X axis) with respect to the image of the mark, to generate first image information and picks up the image of the mark, while scanning the scanning lines in a direction (the direction of the xe2x88x92Y axis) opposite to the predetermined direction (the direction of the X axis) with respect to the image of the mark, to generate second image information, and the calculation device (41) obtains the image information based on the first and second image information.
In the position mea apparatus of the present invention, the image pickup device (32, 51) may be rotatable with respect to the image of the mark.
Moreover, in the position measuring apparatus of the present invention, the image pickup device (32, 51) may include a plurality of scanning lines, of which a first scanning line picks up the image of the mark, while scanning in the predetermined direction (the direction of the X axis) with respect to the image of the mark, and a second scanning line different from the first scanning line picks up the image of the mark, while scanning in the opposite direction (the direction of the xe2x88x92X axis) with respect to the image of the mark.
Furthermore, an exposure apparatus of the present invention has an alignment device (9, 12) which adjusts the position of a substrate (W), based on the position information of a mark (AM, AM1) on the substrate (W), measured by the above described position measuring apparatus, wherein the aligned substrate (W) is exposed with a predetermined pattern.
According to the exposure apparatus of the present invention, alignment of the substrate is performed based on position information detected with high accuracy. Therefore, when exposure is performed repeatedly on a pattern already formed on the substrate, highly accurate superposition is possible, and as a result, fine processing can be realized.